The aim of this proposed research is to more completely understand the role played by the thymus in the differentiation of prothymocytes to T-lymphocytes in fetal, young adult and aged mice. Both in vitro and in vivo techniques will be employed. Stem cells from the yolk sac, fetal liver, young adult bone marrow and aged bone marrow will be isolated by utilizing their propensity to migrate by chemoattraction to media conditioned by newborn thymus or thymic epithelial (T-E) cells. Cells from colonies formed by cultured stem cells stimulated by factors produced by thymic epithelial cells, will be tested for the presence of T-lymphocyte antigenic markers, T-lymphocyte function, and morphologic signs of differentiation. To study the extent to which immunosenescence is a result of the thymus degenerating with age, aged mice will be injected subcutaneously with T-E cells cultured on dextran beads. The effect which factors produced by the T-E cells have on the restoration of T-lymphocyte function will be measured by examining spleen cells for IL-2 production, response to Con A and for cytotoxicity. Bone marrow cells will be tested for their in vitro chemoattraction to thymic supernatant and in vivo migration to the thymus of irradiated hosts. Lymphocytes in the injection site will be examined for thymocyte markers. Athymic mice will be similarly treated and tested for generation of T-lymphocytes. Through this study, the capacity of factors produced by T-E cells to generate T-lymphocyte function will be established. This basic research will be useful in developing rational therapy for disease processes resulting from malformation, malfunction, or aging of the immune system.